Financing Single-family house 520TE current market situation / year-end business banks?

One cannot in good conscience recommend this financing anyway.
I reverse-engineered that about 450,000 EUR are supposed to be borrowed.
Even at 3.45% over 10 years, only 87,000 EUR would be paid off.

Please definitely consider whether you can and want to handle this if the loan really runs for 30 years under these conditions... and whether it is really realistic that you will be able to increase the repayments at some point. As a rule, life does not get cheaper as you get older.

Here one can quote "Herr der Ringe."
Your journey stands on a knife's edge. It is only a matter of a little missing, and it will fail, which means the downfall for everyone.

PS:
Especially when you read gas heating and 80s house...

Just try to get something to be able to start your project. The times have fundamentally changed by now. Even for a normal house with some renovation, only a few can still get a corresponding loan for it. No wonder, as next year an average value decline of 8% is still expected - in exaggeration zones it will be accordingly higher.

Remember the situation from 10-12 months ago - back then a loan of €500,000 would still have been recommended to every household with a household income of €4,500. Still, I would try to get a fixed interest period for as long as possible. Repayment has to be made, the sums for the house + renovation are still too high compared to the cost of money, so it is clear that you have to pay a considerable amount monthly. That might be different next year.

But the interest rates for 10, 15, or 20 years are currently quite close together, a small advantage.

Does anyone here really believe that interest rates will be at 5 or 6% in 10 years?

Do you really believe that interest rates will be back at 2% in 10 years? Did anyone a year ago, at 0.7% for 10 years, believe that only 9 months later the interest would be quintupled? 5-6% interest rates are at least as likely as 2%. It's a gamble. High interest rates have the advantage that the principal repayment portion increases insanely fast due to the saved interest, and financing with 1% repayment becomes healthy again (also only a bit over 30 years term).

Do you really believe that interest rates will be back at 2% in 10 years? Did anyone a year ago believe at 0.7% for 10 years that just 9 months later the interest rate would be multiplied by five? 5-6% interest rates are at least as likely as 2%. It’s a gamble. High interest rates have the advantage that the repayment portion increases insanely fast through the saved interest, and financing with 1% repayment is healthy again (also only just over a 30-year term).

Quite possible. Due to the high interest rates hitting the exorbitant debt of states, companies, and private households, the next financial crisis is already at the starting blocks.

Your theory about repayment and saved interest works when interest rates are high but prices accordingly low... but that is not the case, currently prices are high and interest rates too. That is a toxic mixture, nothing more and nothing less.

Long story short:
Yes, I think interest rates will go down again as soon as inflation decreases. And it will. 8% for several years in a row?
Then we are talking about completely different problems, because 50% of the population would no longer be able to survive.

Yes, I don't want that either. My wife does :) What do you think about the combination with KFW?

What exactly does the combination with KFW look like? And what conditions would be attached to it? The promotional loans also have to be taken onto the banks' books, i.e. if the pure bank loan doesn't go through, neither will the KfW loan. Of course, if you can get a repayment grant or significantly lower interest rate, it can make sense. With the promotional loans I know, the fixed interest rate is always only 10 years.

Do you really believe that interest rates will be back at 2% in 10 years? Did anyone a year ago at 0.7% for 10 years believe that only 9 months later the interest rate would quintuple? 5-6% interest rates are at least just as likely as 2%

Honestly, I also consider 2% or less more likely as a long-term trend. Whether exactly in 9-10 years (when you take care of the follow-up financing) the crisis after next is in full swing again, of course, nobody knows. That's why I find the construct with at least 15 years, but after 10 years you have the right of special termination, quite pleasant, as it gives you a few years to wait (or sort out your own finances) if the situation looks bad at that time.